Fire Safety Journal 59 (2013) 151–159

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Fire Safety Journal

0379-71http://d

n CorrE-m

zangane

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Effect of fuel soaked time and fuel ratio on the flame spread rate over aporous bed wetted with liquid fuel

Jafar Zanganeh n, Behdad MoghtaderiPriority Research Centre for Energy, Faculty of Engineering & Built Environment, The University of Newcastle, Callaghan NSW 2308, Australia

a r t i c l e i n f o

Article history:Received 11 August 2012Received in revised form19 March 2013Accepted 24 March 2013Available online 21 May 2013

Keywords:Flame spread rateLiquid fuelPorous bedFlame initiation delayFuel ratioPermeability

12/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.firesaf.2013.03.017

esponding author. Tel.: +61 249854416; fax: +ail addresses: jafar.zanganeh@newcastle.edu.ah2005@gmail.com (J. Zanganeh).

a b s t r a c t

An experimental investigation was conducted to explore the rate of flame spread over an unconsolidatedporous bed of sand wetted with 2-propanol under a range of operating conditions. Video cinemato-graphy was employed to determine the rate of flame spread and characterise the combustion behaviourof the system. The rate of flame spread strongly correlated with: (i) the ratio of fuel volume to the weightof the sand bed, referred to as FR, and (ii) the flame initiation delay, referred to as FID. The rates of flamespread associated with no initiation delay cases were found to rise with increasing FR while for casesassociated with any given flame initiation delay the rate of flame spread was found to decrease withincreasing FR. In addition, the rate of change in flame spread was observed to be different for bedscontaining finer particles in comparison to those containing coarser ones.

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1. Introduction

Images of large scale fires in industrial settings or residentialareas often express the destructive power of fire and the rate atwhich it can spread to the nearby areas. Fundamental descriptionof flame spread is very complicated given the large number ofunderlying physical, chemical and environmental phenomenainvolved and the numerous variables affecting the combustionprocess at different stages of flame propagation. Among varioustypes of fires, accidental fires in industrial settings, particularlythose in petrochemical industries, are responsible for considerableloss of life and property in many parts of the world every year.Quite often the accidental spillage and subsequent ignition ofcombustible liquids is the cause of many accidental fires in large-scale industrial settings (e.g. refineries). In many such fires theliquid fuel spills over a porous inert surface (e.g. a concrete slab orgravel) and penetrates into its open pores and/or cavities. If thefuel is ignited, a stable flame may form and subsequently spreadover the surface of the porous bed. The characteristics of the flamespread phenomenon over porous beds are significantly differentfrom those over non-porous solids and liquid pools. While thereare many papers on pool fires for example [1–4] and flame spreadover porous bed flushed-up with liquid fuel [5–15], the rate of

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61 249216893.u,

flame spread and flame behaviour over a bed soaked with a finiteamount of liquid fuel is still unexplored. Despite the importance ofthe subject there are only a limited number of studies are availableabout the flame spread over porous beds wetted with finitequantity of liquid fuel in the open literature [16–23].

Ishida [5–8], Takeno and Hirano [15]performed broad experi-mental studies of the flame spread over glass, sands and steelbeads. These studies exclusively focused on scenarios where anunlimited supply of liquid fuel was available to soak the inertporous bed. They found that the flame spread rate is stronglydependent on the size of the solid particles [6]. In addition, it wasfound that the rate of flame spread is generally slower than that ofpool fires.

The porous bed fuel content is a significant parameter in theflame spread phenomenon. The flame spread velocity and flamebehaviour is strongly affected by the initial amount of fuel in theporous bed. For a bed with an unlimited fuel supply, the surface ofthe porous bed is flushed with fuel. In the flushed bed, if a fireoccurs, the flame behaves similarly to a flame on a liquid pool. Themajority of previous studies have been conducted on this type offuel-flushed beds, while in practical situations there is typicallyonly a limited fuel supply. As such the porous bed is not soaked,but rather wetted by the liquid fuel which slowly penetratesthrough the bed. The finite quantity of the liquid fuel in this casehas a significant impact on the flame, altering its propagationbehaviour and/or other characteristics. To narrow down thisknowledge gap we undertook a series of combined experimentaland theoretical investigations at the University of Newcastle

Fig. 2. Schematic representation of the porous bed assembly.

J. Zanganeh, B. Moghtaderi / Fire Safety Journal 59 (2013) 151–159152

(Australia). Results obtained from these investigations revealedthat the flame spread rate under both assisted and opposedairflow configurations noticeably decreased as the air speed wasincreased although the extent of decay in the flame spread ratewas more pronounced in the case of the opposed airflow config-uration [18,21]. However, regardless of airflow speed and/ordirection the rate of flame spread was found to decrease as eitherthe bed depth or particle size are increased [20]. Moreover, theinvestigation revealed that the fuel penetration rate into theporous bed was a function of bed permeability and the mass ofthe liquid fuel. The permeability of the porous bed and conse-quently the rate of fuel penetration were found to decrease aseither the tortuosity or specific surface area were increased [23].

Knowing the flame spread behaviour in fire caused by fuelspillage will help to take proper action to control and suppress thefire at the beginning of combustion process.

Despite the importance of the flame propagation, particularlyfrom fire safety point of view, however, this phenomenon still isnot well understood. The present study builds upon our previousinvestigations [16–23] specifically aims at examining the effect offuel ratio (FR) and flame initiation delay (FID) on the rate of flamespread over a solid porous bed wetted with a limited quantity ofliquid fuel. To achieve the broad objectives of the study acomprehensive series of combined experiments and theoreticalinvestigations were carried out. The details of these are discussedbelow.

2. Apparatus and procedure

A series of laboratory scale experiments were performed usingthe experimental setup shown schematically in Figs. 1 and 2.A channel with scaled length in centimetre was designed to mimicthe spread of flames over a range of porous media under realisticfire scenarios under controlled laboratory settings (Fig. 1). The150�8�4 cm3 channel (Fig. 2) was made out of stainless steeland mounted on a support frame capable of adjusting the angle ofinclination of the channel. The external surfaces of the channelwere covered with a 5 mm thick insulating material (calciumsilicate sheet) to prevent heat losses. The bottom of the channelwas fitted with a cooling system to maintain its temperaturearound the ambient temperature. The cooling system consisted ofa set of copper tubes connected to a water bath, a water circulationunit and a digital controller. The channel was also fitted with anarray of type K (Ni/Cr–Ni/Al, −200–1370 1C) thermocouples fortemperature measurements. The channel was filled with sand(ρ¼2680 kg/m3) as the porous bed. Six different particle sizes of

Fig. 1. Experimental set up.

sand, namely 0.5, 1, 2, 3, 4 and 5 mm were used. In addition, threedifferent depths (13.3, 26.6 and 39.9 mm) were selected toinvestigate the effects of bed depth on the flame spread phenom-ena and the overall combustion process. To minimise the impact ofambient temperature on the thermocouple reading, thermocou-ples were embedded just beneath the surface (y¼−1 mm) of thebed. The ambient temperature in the laboratory was also regulatedat 25 1C during the experimental campaign.

A canopy hood connected to an exhaust fan was installed abovethe bed to vent the combustion products out of the laboratory. Thecanopy hood was switched off at the beginning of each experi-mental run to minimise the effect of induced air on the rate offlame spread as well as combustion process.

2-propanol (boiling point 82.4 1C and flash point 12.7 1C) wasused as the liquid fuel in all experiments. A finite quantity of theliquid fuel was uniformly distributed in a defined constant timeframe over the top surface of the porous bed by a custom madefuel distributor. The ratio of fuel volume to the weight of the sandbed (referred hereafter as “Fuel Ratio, FR”) was set to 0.1, 0.125,and 0.15 (L/kg) in the experiments. These ratios were calculatedfrom the proportion of the solid bed volume/mass to the total bedvoidage. The bed voidage in this study was assumed to underlay ina range of 0.39–0.44 [23,24]. Accordingly, the fuel ratio for a halfflushed bed with considering 720% for upper and lower rangesare 0.1, 0.125, and 0.15 (L/kg).

Ignition was achieved using a pilot flame placed near the end ofthe channel (displayed by “A” letter in Fig. 1). A time delay (referredto as “Flame Initiation Delay, FID”) was maintained between thecompletion of the fuel distribution and the onset of the ignitionprocess. Three different flame initiation delays of 0, 10 and 15 minwere used in our experimental studies. In order to minimise the fuelevaporation to the lowest extent, after completion of fuel distribu-tion, the channel surface was covered and nearly sealed with a lidconstructed as part of the set up. The lid was used for theexperiments with FID40 only. In order to maintain the consistencybetween different set of experiments (i.e. FID¼0, and FID40) the lidwas removed from the top of the channel 1 min before flameinitiation. This helps to eliminate any possible gaseous vapouraccumulated underneath lid during the FID period.

The flame speed and hence the spread rate were digitallydetermined from video (JVC-HDD brand, capable of taking 25frames per second) cinematography, video image processing(Prism video convertor and Ultra video editor programs) andvisual observation.

3. Results and discussion

3.1. Flame spread rate for different FRs and FIDs

The flame spread rates obtained under quiescent conditions inthe horizontal orientation for a range of FRs and FIDs are shown in

Fig. 3. Flame spread rate over porous beds containing 0.5 mm sand particleswetted with 2-propanol at different FRs and FID¼0 min.

Fig. 4. Flame spread rate over porous beds containing 0.5 mm sand particleswetted with 2-propanol at different FRs and FID¼15 min.

Fig. 5. Flame spread rate over porous beds containing 5 mm sand particles wettedwith 2-propanol at different FRs and FID¼0 min.

Fig. 6. Flame spread rate over porous beds containing 5 mm sand particles wettedwith 2-propanol at different FRs and FID¼15 min.

J. Zanganeh, B. Moghtaderi / Fire Safety Journal 59 (2013) 151–159 153

Figs. 3–8 and Table 1. Figs. 3 and 4 show bar charts of the flamespread rate plotted as a function of the bed depth for FR values of0.1, 0.125, and 0.15 L/kg. The figures clearly show that the rate offlame spread depends on the liquid fuel ratio (i.e. FR), flameinitiation delay (i.e. FID), bed depth, and particle size.

As shown in Fig. 3, in the case of flame initiations without delay(i.e. FID¼0) the rate of flame spread for the bed containing thefinest particle size (0.5 mm) increases as the bed depth decreases.Moreover, the rate of flame spread increases as FR is increased. Thehighest flame spread rate is about 1.63 m/s and corresponds to thebed with a depth of 13.3 mm and FR¼0.15 L/kg. For the same beddepth the rate of flame spread decreases to 1.563 m/s and 1.5 m/s,respectively, when the FR is lowered to 0.125 and 0.1 L/kg. Asimilar pattern is observed for deeper beds as well. For the deepestbed (39.9 mm) the rate of flame spread decreases from 1.5 m/s to1.388 m/s as FR decreases from 0.15 to 0.1 L/kg. For the same FR,the flame spread rate over a bed depth of 39.9 mm shows a drop ofalmost 8% when compared with the flame spread rate over a13.3 mm bed depth.

Unlike the flame initiation with no delay (Fig. 3), in the case ofFID¼15 min the rate of flame spread decreases as the fuel ratio is

increased. This is clearly illustrated in Fig. 4 where the rate offlame spread has been plotted versus bed depth for a 0.5 mmparticle size bed wetted with different FRs. It can be seen from thisfigure that regardless of the values of FR and FID the rate of flamespread decreases almost linearly with increasing bed depth.Results corresponding to the 15 min flame initiation delay indicatethat the highest flame spread rate belongs to the smallest FR,namely 0.1 L/kg. For instance, for the 13.3 mm bed depth the rateof flame spread decreased from 1.44 m/s to 1.39 m/s and 1.34 m/sfor the FRs of 0.1, 0.125, and 0.15 L/kg, respectively. The lowestflame spread rate was approximately 0.87 m/s for the deepest bed(39.9 mm) and the largest FR (0.15 L/kg).

It may be inferred that the rate of flame spread plummetswhen both FR and FID are increased. For the FID¼0 min case therate of flame spread corresponding to smaller FRs are slightlylower than those of larger FRs, while in the case of FID40 (e.g.FID¼15 min) the rate of flame spread is inversely proportionalwith FR and shows signs of deceleration as FR is increased. There isa clear distinction between the results presented here and thosestudies where an unlimited supply of liquid fuel is available in theporous bed. For instance, a former study by Ishida [6] has shown

Fig. 7. Flame spread rate over a porous bed of 13.3 mm depth containing sand particles ranging from 0.5 mm to 5 mm at different FRs and FIDs.

Fig. 8. Flame spread rate over a porous bed of 39.9 mm depth containing sand particles ranging from 0.5 mm to 5 mm at different FRs and FIDs.

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that when glass beads are soaked with liquid fuel (n-octane), theelapsed time from the fuel spillage to ignition does not affect therate of flame spread. Ishida's results showed that regardless offlame initiation delays (ranging from 1 to 10 min) the rate of flamespread was almost constant. The difference between the currentresults and those of Ishida [6] is largely related to the interplaybetween the FID and the finite quantities of fuel that we used inour studies (a more detailed discussion about FID and its impact ispresented in latter parts of this section). This approach is morerealistic since in practical situations an unlimited quantity of liquidfuel is not usually spilled over a porous bed.

Figs. 5 and 6 show the rate of flame spread over porous bedscontaining a coarse monotonic sand particles size of 5 mm under arange of operating conditions. In these figures the rate of flamespread has been plotted versus the bed depths for different FRsand FIDs. Overall, the trends shown in these two figures are verysimilar to those presented earlier in Figs. 3 and 4; and confirm thatfor any given FR and FID the rate of flame spread decreases as thebed depth increases. However, the impact of the bed depthparticularly at FID¼15 min appears to be more pronounced inthe case of 5 mm sand particles if one compares Figs. 4 and 6. Forinstance, when the 5 mm particles bed is 13.3 mm deep the flame

Table 1Flame spread rate over various porous beds for different FIDs and FRs.

Bed depth(mm)

FR(L/Kg)

Flame spread rate (m/s)

FID¼0 FID¼0 FID¼10 FID¼10 FID¼15 FID¼15PS¼0.5 PS¼5 PS¼0.5 PS¼5 PS¼0.5 PS¼5

13.3 0.1 1.5 1.389 1.47 1.339 1.442 1.250.125 1.563 1.442 1.442 1.293 1.389 1.210.15 1.63 1.5 1.389 1.250 1.339 1.172

26.6 0.1 1.47 1.25 1.442 1.21 1.389 0.9150.125 1.5 1.293 1.39 1.172 1.339 0.8930.15 1.563 1.339 1.339 1.154 1.293 0.872

39.9 0.1 1.388 1.071 1.293 0.872 1.21 0.6580.125 1.442 1.21 1.25 0.852 1.172 0.6360.15 1.5 1.293 1.119 0.852 0.872 0.605

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spread rate decreases from 1.25 to 1.17 m/s as FR increases from0.1 to 0.15 L/kg (see Fig. 6). This is a reduction of about 7% butunder almost identical conditions the drop in the flame spreadrate for the 0.5 mm particles bed is just 5% (see Fig. 4). The particlesize of the porous bed is obviously having an impact on the rate offlame spread. Comparing the flame spread results correspondingto the beds containing 0.5 mm and 5 mm sand sizes reveals that ingeneral the rate of flame spread is distinctively smaller for coarserparticles.

Figs. 7 and 8 reveal the impact of particle size more clearly.In these figures the rate of flame spread has been plotted againstparticle size (ranging from 0.5 mm to 5 mm) for a single bed depth(13.3 mm in Figs. 7 and 39.9 in Fig. 8). The results have beenpresented as a function of FR and FID. As shown in Fig. 7, for a bedof 13.3 mm depth with FID¼0 min, the flame spread rate gradu-ally decreases as the particle size increases. The highest flamespread rate belongs to the FR¼0.15 L/kg over the bed containing0.5 mm sand particles. A similar pattern is observed for thesmaller FR values of 0.125 L/kg and 0.1 L/kg.

Like the flame spread without initiation delay, in the case of 10and 15 min delays the flame spread rate decreases with increasingparticle size. But, for these FIDs the rate of flame spread isevidently higher for smaller FRs. Furthermore, the rate of flamespread decreases gradually as either FR or FID is increased. Therate of flame spread over a bed containing a particle size of 0.5 mmand a fuel ratio of 0.15 L/kg shows that the flame spread ratedecreases from 1.39 m/s to 1.34 m/s for 10 and 15 min flameinitiation delays, respectively. These values show a drop ofapproximately 15–20% in the rate of flame spread when comparedto corresponding values for flame initiation with no delay.

Fig. 8 shows the rate of flame spread over the deepest porousbed (39.9 mm) for different FIDs and FRs. In comparison with thebed depth of 13.3 mm it can be seen that under equivalentconditions the flame spread rate for the 39.9 mm bed depth isslower for all particle sizes. From this figure it is inferred that therate of flame spread is inversely proportional to the particle sizeand FID.

The above figures also highlight the significant impact of theFID on the rate of flame spread. Figs. 3–8 clearly point out that,regardless of the bed depth, when finite quantities of liquid fuelare available at the combustion zone (i.e. bed surface) the rate offlame spread decreases as FID is increased. Moreover, with theexception of FID¼0 min, the rate of flame spread for larger valuesof FR decreases as the FID increases.

3.2. Effect of FIDs on the rate of flame spread

The effect of FID on the rate of flame spread can be bestexplained in terms of the interaction among intermolecular forces.

Generally, the mechanism causing the fuel to rise to the surface ofthe bed or sink to the bottom of the bed is governed by theinteraction among intermolecular forces (e.g. surface tension,cohesion, adhesion) acting between the molecules of the liquidfuel and solid particles as well as the fluid flow through the porousstructure of the bed.

The cohesive force tends to hold the molecules of fuel together,while the molecules of fuel are attracted towards the sandparticles by adhesive forces and surface tension. The cumulativemagnitudes of these intermolecular forces strongly act against theforce of gravity and contribute to the capillary rise. When fuelspills over a porous bed the liquid fuel molecules form a thin layeron the surface of the bed due to cohesive and adhesive forces. Forbeds consisting of small particles more fuel remains on the topsurface of the bed since in this case the adhesive and cohesiveforces are much stronger due to the small space between adjacentparticles. In contrast, for beds containing large particles theadhesive forces between liquid and solid molecules are weakerdue to larger spaces involved, and as a result the fuel can penetrateand sink more easily into the bed.

Apart from the bed particle size, the balance between theattractive forces and gravity is also affected by the quantity of fueldistributed over the bed (i.e. FR). For a given particle size, when FRis small the attractive forces tend to balance the effect of gravity.This may manifest itself through the formation of a liquid film onthe top surface of the bed (our visual observations indeedconfirmed the existence of this liquid film). At higher FR values,the weight of the liquid layer can no longer be sustained byattractive forces and thus the fuel sinks into the bed.

Regardless of what causes the imbalance between the attrac-tive forces and gravity (i.e. whether it is particle size or FR) the endresult is the penetration of the liquid fuel into the bed andultimately a level of fuel distribution within the bed. To a largeextent, the distribution of fuel and its availability near the surfaceof the bed determines the flame spread rate, underpinning theresults shown in Figs. 3–8. However, the fuel penetration processis a time dependent and thereby transient phenomenon whichstrongly depends on FID and the nature of the fluid flow throughthe porous bed. For short FIDs (FID∼0 min) the extent of penetra-tion and subsequent fuel distribution within the bed may not belarge and hence more fuel would be available at the surface of thebed to support rapid flame propagation. In contrast, when FID isrelatively large, there is ample time for the penetration phenom-enon to develop; starving the flame from sufficient quantities offuel. This, in turn, would result in slower flame spread rates asobserved in Figs. 4 and 6. The fluid flow through a porous mediumlike a bed of sand is quite complicated and governed by a complexset of physical phenomena. The mechanism of fuel flow throughthe porous medium is influenced by the kinematic and dynamicrelationships of the liquid fuel, porous medium, and fluid flow. Theporous bed made up of sand consists of a large number of voidspaces of various sizes. The bed can be considered as a bundle oftortuous capillary channels with variable cross sectional areas[23,25]. Generally, the permeability of the bed is underpinned bythe size, number, and tortuosity of these channels which them-selves are dependent on the geometry of particles and the type ofprevailing packing in the bed, i.e. loosely packed versus denselypacked [26]. For beds constructed of small particles, the sizes ofthese channels are usually smaller in comparison with largerparticles. In addition, the number of channels and particularlythe number of open channels are dependent on the porous bedconfiguration. In the case of a dense packed bed a large number ofthe interconnected narrow channels are compacted or blocked.However, in the case of loosely packed beds most of thesechannels remain open [27]. The permeability of the porous bedhas a significant impact on the rate and ease of fluid flow through

J. Zanganeh, B. Moghtaderi / Fire Safety Journal 59 (2013) 151–159156

the porous bed. This parameter is significantly related to the grain-size distribution and the way that the bed has been formed. Giventhat the pressure drop is the driving force behind any fluid flow,the flow of the liquid fuel through a porous medium such as a bedof sand can be described using Darcy's equation [28] and Kozeny–Carman equation which takes the permeability into account [29].

Q ¼ −kAμ

ΔPL

ðDarcy’s equationÞ

ΔpL

¼ 150 V0μ

Φ2s D2

P

ð1−εÞ2ε3

ðKozeny−CarmanÞ

where Q is volumetric flow rate, A bed cross section, k perme-ability, μ viscosity, ΔP pressure drop, L bed depth, Dp particlediameter, ε Porosity, Φ particle shape factor (sphericity).

Typically, the formation of the porous bed (i.e. packing type[24]) and the geometry of particles can create obstacles on the bedsurface which may cause interruption to the flame spread process.As the flame leading tip spreads on the bed surface, it heats up thebed surface and also unburned fuel ahead. If a barrier or a largeroughness (bump) is encountered it can rupture the flame spreadand retard or even stop the flame propagation process completely.Studies by Hirano et al. [30] and Ishida [31] have shown that thedetention time at the barrier is strongly depends on the height andthickness of the obstacle. Barriers with height more than 10 mmcan temporarily stop flame progress for a second; the flame thenoverruns the obstacle. For thicker and higher obstacles, however,the detention time will be longer. In the present study byassuming the bed formation of random packing of mono-sizespheres either in orthorhombic or tetragonal modes [24] themaximum height for any sort of obstacle on the surface for thebed containing 5 mm sand particles is approximately 2.5 mm.Therefore, during the flame spread experiments no flame deten-tion was observed. Moreover, in comparison with results obtainedfrom the studies by Ishida and Hirano this height (2.5 mm) is notconsidered as a barrier, thereby the effect of bed surface roughnesson the rate of the flame spread is negligible.

3.3. Controlling mechanism of flame spread over liquid pool

Pulsating flame spreading over liquid fuels is a classic problemof combustion theory whose mechanism is still a controversialsubject. Flame spreading over liquid fuels is usually accompaniedby liquid fuel motions generated by the thermo-capillary effect[32,33]. At sufficiently high temperatures, vaporisation of theliquid fuel leads to formation of a flammable gaseous layer. Uponignition, a flame may propagate in this layer without furtherheating or vaporisation of the liquid.

Experimental studies have shown that the flame spreadmechanism on liquid fuels mainly depends on the initial liquidtemperature being above or below the flash point. When the liquidtemperature is above the flash point, a flammable mixture isestablished over the fuel surface and flame spread is believed to becontrolled by gas phase-liquid phase convection effects, since itoccurs only when subsurface liquid convection has been generated[34–36]. Upon flame propagation the liquid fuel temperatureincreases due to liquid phase preheating. The preheating of thefuel is mainly due to radiation and heat conduction from the flamethrough the gas phase [37].

The heating of the liquid fuel under the flame tip producessurface tension gradients that can lead to motions in the liquidbulk by viscous stresses. This is a distinctive characteristic withrespect to flame spreading above a solid fuel and provides amechanism to promote the flame spreading. Akita [38] stated theexistence of different regimes exhibited by a flame spreadingabove liquid pool. He found that in the uniform region, the flame

spread rate is governed by liquid phase convection. In addition, heobserved that for large values of the initial surface fuel tempera-ture the fuel vapour pressure is very high, and leads to a flamespreading regime purely controlled by the gas phase. Flamepropagation velocities at this region observed approximately1 m/s. For relatively high temperatures the flame spreadingvelocity is controlled by heat diffusion in the condensed phase(similar to the solid case). As the initial liquid temperature isdecreased from the flash point, flame motion turns from uniformspread to pulsating spread and then, for even lower temperatures,to uniform spread again [35,39]. Flame spread velocity decreasesalmost linearly with decreasing temperatures [32,40,41]. Uponincreasing temperature beyond the liquid flash point, the flamespread propagation continues through the premixed fuel and air inthe gas phase.

In contrast, in porous bed thermal conductivity and thermo-capillary are considered as controlling parameters of flame spread.Thermal conductivity is the property of a material to conduct heatfrom the flame to the fuel whilst thermo-capillary effect refers tothe spatial variation of surface tension resulting from the non-uniformity of temperature on the gas-liquid inter-phase. There aretwo interpretations, in general, for the effect of thermal conduc-tivity enhancement on the rate of flame spread. In one hand it issupposed that by increasing the bed thermal conductivity, due toan increase in heat transfer from the flame to the fuel, the rate offuel evaporation increases resulting quicker flame spread. But, onthe other hand by increasing the thermal conductivity the heatloss increases, as such the evaporation rate and consequentlyflame spread rate decreases [15,42–46]. Unlike the normal condi-tion, when the porous bed temperature is far above the liquid fuelflash point, variations of porous bed thermal conductivity has nosignificant effect on the rate of flame spread [6].

There has been a considerable number of studies that haveinvestigated the thermo-capillary effect on the flame spreadingover liquid fuel surface. For instance, Higuera [47] examined theeffect of thermo-capillary flow, induced in a liquid layer by asurface heat flux distribution, on the flame spreading over theliquid surface. He observed the formation of three differentregimes which correspond to the uniform, pulsating and pseudo-uniform regimes of flame spread. These regimes usually formedwhen the ambient temperature of the liquid, or the strength of thesurface heat flux, is decreased. The uniform and pseudo-uniformare stationary regimes of high and low front speed, and thepulsating is an oscillatory regime featuring long phases of lowspeed and short pulses of high speed. Results obtained by otherresearchers such as Akita, Glassman, etc. also show similar results[35,38,40,47–50].

3.4. Controlling mechanism of flame spread over porous bed

Generally, there are two modes of flame propagation control-ling mechanisms; (i) liquid phase controlled propagation and (ii)vapour phase controlled propagation. The liquid phase controlledmechanism is referred to the situation when the liquid tempera-ture is below its stoichiometric temperature, while a vapour phasecontrolled condition exists when the liquid temperature is abovethe fuel stoichiometric temperature.

The stoichiometric temperature for the combustion of liquidfuel is referred to the temperature at which the fuel vapourpressure is sufficient to form a vapour fuel/air mixture layer forcombustion reaction. For the porous bed where the temperature isrelatively lower than the spilled liquid fuel stoichiometric tem-perature, the flame propagation is relatively slow. However, whenthe bed temperature is higher than the stoichiometric temperaturethe flame leading tip travels throughout the bed surface due to the

Fig. 9. Effect of FR on the rate of fuel penetration for a range of sand beds withparticle sizes ranging from 0.5 mm to 5 mm (39.9 mm).

Fig. 10. Plot of penetration depth versus time for different particle sizes atFR¼0.1 L/kg.

J. Zanganeh, B. Moghtaderi / Fire Safety Journal 59 (2013) 151–159 157

existence of sufficient fuel-air mixture, and thereby the bed isenveloped with a large flame pillar in a very short time.

In the present study the bed temperature is maintained aroundambient temperature. This temperature is approximately betweenthe liquid fuel flash point and the stoichiometric temperature.Therefore the flame propagation over the bed at different stages isconsidered to be controlled by both fuel vapour/air mixture andthe liquid phases. Since the porous bed is being wetted with finitequantities of liquid fuel (i.e. different fuel ratios) and flame isinitiated at different time spans, therefore, the influence of each ofthese controlling mechanisms may differ under different condi-tions. For instance for the bed wetted with the larger fuel ratio ifthe flame initiated immediately after fuel spillage (FID¼0) therewill not be sufficient time available for the fuel to penetrate in thebed, therefore, a thin layer of liquid fuel remains above the surfaceand behaves like a pool fire. As such, the liquid phase controls thepropagation mechanism. For the FIDs40, however, the fuelgradually penetrates deeper to the bed and pools at the bottomof the channel. Accordingly, as the combustion proceeds the heatis conducted into the bed and enhances the fuel evaporation rate.The vapour fuel then diffuses through the bed and a fuel/airmixture layer is formed above the bed surface. The thickness andconcentration of the vapour fuel layer is controlled by diffusiontime and liquid temperature, respectively [51,52]. For FID40,nevertheless more fuel vapour is generated, however, in the meantime some fuel vapour is dispersed to the ambient. As such, thevapour fuel/air mixture concentration gradient above the bedsurface remains nearly constant.

Kaptein and Hermance [53] investigated the controllingmechanism of flame propagation over a porous bed. They used achannel filled with liquid fuel up to certain point. Then a wiremesh screen placed above the fuel level to separate the fuel fromthe glass bead particles. The glass beads were dry and there wasno contact between liquid fuel and glass particles. They found thebed temperature gradually is transferred to the fuel and evaporatethe fuel. Then the flammable gas diffuses up to the surface throughthe particles and controls the flame propagation. Unlike Kapteinand Hermance [53], in the present study, however, it appears thatfor FID40 the flame propagation is controlled by a combination oftwo liquid phase and vapour phase mechanisms. Even though, itseems that the influence of fuel vapour/air mixture phase is morepronounced.

3.5. Fuel penetration time for different FRs

In an attempt to quantify the impact of FID on fuel penetration(and hence flame spread rate) we carried out a set of experimentsusing a modified version of the experimental setup described inSection 2. In this setup a transparent channel was employed so thatthe transient behaviour of the fuel could be monitored as the fuelpenetrated through the sand bed. The experimental data collectedfrom these experiments were then compared with theoretical resultsobtained from Kozeny–Carman based equation. The full descriptionof our theoretical analysis and the experimental procedure fordetermining the fuel penetration depth can be found in one of ourearlier publications and are not repeated here [16,23].

Results corresponding to the rate of fuel flow through theporous bed as a function of particle size and FR are shown in Fig. 9.In this figure the rate of fuel flow through the porous bed for bothpredicted and experimental results are plotted versus the bedparticle size for sizes ranging from 0.5 mm to 5 mm. As shown,there is a good correlation between the predictions and experi-mental data. Both sets of results indicate that the rate of fuel flowthrough the bed increases significantly as the particle size isincreased. The rate of fuel penetration appears to be higher forbeds containing coarse particles. This can be partly explained in

terms of the bed permeability which as noted earlier is higher forporous beds comprising coarse particles.

Results corresponding to the fuel penetration depth versus timefor a FR¼0.1 L/kg are shown in Fig. 10 for a range of particles sizes.As can be seen from Fig. 10, depending on the particle size the valueof penetration time (PT) ranges between 2 and 42 s, meaning that itwould take anywhere between 2 and 42 s for the fuel front to reachthe bottom of the bed. These values which are much shorter thanflame initiation delays of 10 min and 15 min implemented in some ofour experiments, implying that when FID ⪢ 0 much of the fuel hasalready moved away from the top surface of the bed. This, in turn,suggests that for FIDs greater than zero the flame spread is not astrong function of penetration time. More importantly, for FID ⪢ 0the quantity of the fuel near the surface and in the vicinity of theflame may not be able to support rapid flame propagations. Indeed acomparison of Figs. 3 and 4 or Figs. 5 and 6 show that flame spreadrates associated with FID¼15 min are generally lower than theirFID¼0 min counterparts.

J. Zanganeh, B. Moghtaderi / Fire Safety Journal 59 (2013) 151–159158

Unlike FID ⪢ 0 cases, when FID¼0 min the flame spread ratewould inevitably be a function of penetration time (PT) becausethe amount of fuel penetrated through the bed is dependent on PT.The higher the PT the slower the fuel penetration process and,thereby, the higher the quantity of the fuel near the top surface ofthe bed. It is, therefore, not surprising to see that the flame spreadrate attains high values for smaller particles sizes and deeper beds(see for example Figs. 7 and 8).

4. Conclusions

An experimental study was performed to examine the flamespread rate over an unconsolidated porous bed of sand wettedwith 2-propanol under diverse conditions. The experimentalresults presented in this study show that the flame caused fuelspill can propagate in laminar mode through thin layer of liquidfuel film with small presence of fuel/air mixtures. The speed of theflame along the bed depends on the quantity of liquid fuel and alsoburning velocity of the stoichiometric mixture of fuel vapourwith air.

Generally when the flame is initiated with no delay, the rate offlame spread over porous bed wetted with finite quantities ofliquid fuels is higher for larger fuel ratios in comparison withsmaller fuel ratios. However, for flame initiation delays greaterthan zero the rate of flame spread decreases as the fuel ratioincreases. The rate of flame spread depends on the fuel ratio whenthe flame initiation delay is smaller than required time forcomplete fuel penetration. Moreover, the rate of flame spread isinversely proportional to the bed depth; therefore, regardless offuel ratio and/or flame initiation delay the rate of flame spreaddecreases as the depth of porous bed increases. The flamepropagation is controlled via liquid phase and vapour phasemechanisms. In the case of flame initiation with no delay theflame propagation is mainly controlled by the liquid phase, whilefor the flame initiation with some delay the vapour phase isconsidered to be the dominant mode of controlling mechanism.

The fuel penetration velocity and depth are considered assignificant factors on the rate of flame spread over the porousbed. The fuel penetration velocity and depth considerably dependson the fuel ratio, permeability as well as bed characteristics. Thefuel penetration velocity and depth of penetration increase as thefuel ratio and initiation delay increase, respectively.

Acknowledgements

The authors wish to thank Prof. Hiroki Ishida (Nagoya NationalCollege of Technology-Japan) and Mr. Neil Alston (The Universityof Newcastle-Australia) for collaboration and constrictive supportin this study. Moreover, the authors like to acknowledgethe financial support provided to them by the University ofNewcastle (Australia).

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